1. Field of the Invention
This invention relates in general to the manipulation of light waves for the purpose of transmitting, receiving and processing digital information, and more particularly to the modulating, detecting and performing of logical operations with coherent light.
2. Description of the Prior Art
It has been known for some time that the use of light as a medium for the transmission of information has distinct advantages over conventional means, primarily because of the tremendous bandwidth available at such short wavelengths. With the introduction of light emitting diodes and more particularly of lasers, the technology became a reality and now data transmissions at the rate of one billion bits per second can be sent from point to point over optical fibers many miles in length. The highest transmission rate achieved thus far is about twenty billion pulses per second, accomplished by multiplexing several lasers operating at different wavelengths. Even this rate, however, is but a fraction of the theoretical maximum for a light source operating, for instance, at a wavelength of 900 nanometers. The primary limiting factor in the transmission rate is the frequency at which a light source (typically a light emitting diode or a laser) can be turned on and off. The apparent maximum is about one billion hertz. The present invention does not turn a laser on and off, but modulates the light by an indirect method.
Likewise, detecting high repetition rate light pulses is difficult, and has about the same upper limit with devices now in use (PIN diodes and avalanche photodiodes, etc.). The present invention has a means of detecting high rate, low power light pulses.
It has also been known for some time that if a way could be found to use photons, as opposed to electrons, as a basis for devices which can perform logical operations, that such devices would have substantial advantages (primarily in processing speed) over conventional devices. These logical operations; logical AND, logical OR, INVERT, and EXCLUSIVE OR are the building blocks of most digital electronic circuits.
An optical logic device would preferably be subject to miniaturization and high density packaging, have quantized inputs and outputs, and lend itself to inexpensive mass production. The invention described herein performs extremely high speed logical operations, can be made to interface with most digital devices presently in use, has highly quantized inputs and outputs, and can be mass manufactured in high density packaging with currently available photolithographic techniques.
Many optically bistable devices have been proposed, and several have recently been constructed. All take advantage of the nonlinearity in the refractive index of certain mediums, and most make use of a tuned cavity, or interferometer, in which the refractive index is changed by electrical or electromagnetic energy injections. I believe that it is unnecessary to confine coherent light in a tuned cavity in order to exercise control over the phase and amplitude of the light. If the phase of one-half the photons in any beam of coherent light can be changed with respect to the other half, the amplitude and phase of the composite beam can be controlled. Moreover, the use of a tuned cavity incorporates some of the same problems that plague the pulsed laser itself. That is, the speed at which it operates depends to a great degree on the time it takes for the relections inside the cavity to die out after the transition from on to off. The length of one bit of information is dependent on the optical length of the cavity. The operation of these types of devices, in most instances, employs a reference beam to hold the cavity just below a threshold level, and a second modulated probe beam to push the cavity past the threshold, which exponentially increases the output. This, however, does not address the original problem of how to increase the modulation frequency of the probe beam.
The present invention does address this problem and is not restricted by the liabilities of a tuned cavity. A tuned cavity or interferometer is also extremely temperature sensitive. Since the present invention relies on complimentary halves of the same crystal, phase changes due to temperature variations are the same in both halves. A tuned cavity also requires a full 180.degree. phase shift in the cavity in order to achieve true bistability (90.degree. change in the optical length of the cavity). In the present invention, any unit of phase shift can be considered a transition from an absolute logical low to an absolute logical high.